Though a styrenic resin has heretofore been widely used in a variety of fields as a general-purpose resin, it has suffered the serious disadvantage of its inferior impact resistance in general. In order to improve the impact resistance of a styrenic polymer, there is industrially produced a so-called rubbery modified styrenic polymer composition, that is, a substantial mixture of a rubbery polymer-styrene graft copolymer and polystyrene in which a part of styrene is graft-polymerized onto a rubbery polymer and the remainder thereof constitutes polystyrene by a method in which styrene is blended with a rubbery polymer or a method in which styrene is polymerized in the presence of a rubbery polymer.
In particular it is known that the impact strength of a styrenic resin composition is enhanced by the use of a perfectly block-type styrene/butadiene copolymer or a taper block-type styrene/butadiene copolymer. For example, there are disclosed an impact-resisting polystyrene in which a perfect block-type styrene/butadiene copolymer is used as the rubbery polymer (refer to Japanese Patent Application Laid-Open No. 165413/1988) and an impact-resisting polystyrene in which a taper block-type styrene/butadiene copolymer is used as the rubbery polymer (refer to Japanese Patent Application Laid-Open Nos. 71549/1977 and 48317/1988).
However, it can not be said that the above-disclosed impact-resisting styrenic resin is always sufficient in the balance among impact resistance, heat resistance, mechanical strength, etc., and thus a styrenic resin which is highly balanced in the aforesaid properties is desired according to the purpose of use thereof.
Being produced by radical polymerization, the generally used styrenic resin is of atactic configuration in its stereoregularity and noncrystalline. Accordingly it can not be said that such styrenic resin is sufficiently high in impact resistance and mechanical strength, thus limiting the improvement in these properties.
In order to develop a styrenic resin composition which has further excellent properties beyond the limit of the improvement in properties inherent in the conventional styrenic resin, the group of the present inventors continued research and proposed a highly impact-resistant styrenic resin composition which comprises a styrenic polymer having high degree of syndiotactic configuration compounded with a rubbery polymer having a styrenic unit as one component (Refer to Japanese Patent Application Laid-Open No. 146944/1989).
Nevertheless, since there has been used in the above-mentioned composition, a rubbery polymer having a product of weight-average molecular weight and styrenic monomer unit content of less than 30,000, the composition has suffered the disadvantages that it brings about a decrease in rigidity at the time of molding presumably attributable to poor dispersion of the rubber and insufficient release properties; and besides the molding produced therefrom is not sufficiently improved in Izod impact strength with low heat resistance (Vicat softening point).